1. Field of the Invention
This invention relates to a method of manufacturing an improved oxygen senser.
2. Description of the Prior Art
Heretofore, it has been known to use an oxygen senser which is operative to detect an oxygen concentration in exhaust gas delivered from an internal combustion engine or a combustion engine for boilers for the purpose of controlling a ratio of air to fuel in a mixed gas to be supplied to the combustion engine. The oxygen senser is composed of a detection element formed of an oxygen ion transmissible metal oxide. The detection element is provided at its inner surface exposed to a reference gas (e.g. atmospheric air) and at its outer surface exposed to a detection gas (e.g. exhaust gas ) with respective electrodes each formed of a this electrically conductive film (e.g. a platinum film) deposited thereon by non-electrolytic plating, electrolytic plating, physical treatment or the like. An electromotive force induced in the detection element is taken out of the electrodes and the difference between the oxygen partial pressure of the reference gas and the oxygen partial pressure of the detection gas is detected so as to control the ratio of air to fuel.
The oxygen senser constructed as above described must satisfy the following conditions.
(1) The electrodes must not be peeled from the detection element even when the electrodes are exposed to the detection gas at a high temperature for a long time.
(2) The oxygen senser must have a sufficiently high response property and hence must have a sufficiently large three phase boundary surface and must maintain its high response property even when it is used under severe conditions.
(3) The oxygen senser must have a stable output whose value is not shifted toward the negative side. The conventional method of manufacturing the oxygen senser heretofore proposed could not satisfy the above mentioned conditions (2) and (3). An attempt has been made to make the diameter of holes or the width of broken holes produced when the electrodes are heat treated smaller than a size corresponding to the thickness of the electrodes. Inventors' experimental tests, however, demonstrated the result that the use of the hole or broken hole having such diameter or width decreases the three phase boundary surface to degrade the characteristic, particularly, response property of the oxygen senser as shown in Example 1 in the following Table 1. The cause of this decrease of the three phase boundary surface is not clear, but would be attributable to change its quality or recrystallization of constitutional metal of the electrode or to the fact that the hole or broken hole is clogged with certain ingredients in the exhaust gas.
TABLE 1 ______________________________________ Average Dia- Response Property (msec) Film meter or Width In the Thick- of Hole or Begin- After After ness (.mu.) Broken Hole ning 100 hr 500 hr ______________________________________ Exam- Immeas- Immeas- ple 1 About 1 0.3 420 urable urable Exam- ple 2 About 1 0.7 130 200 370 Exam- ple 3 About 1 5 80 60 60 ______________________________________
In the above Table 1, the response property shall be understood to mean time required to decrease the output from the oxygen senser from 600 mV to 300 mV.